The present invention relates to ink jet printers of the type which deposit ink drops from one or more jet drop streams on a print receiving medium and, more particularly, to a printer arrangement in which start up of the printer is enhanced.
Typically, an ink jet printer includes a print head defining a fluid reservoir to which electrically conductive ink is supplied under pressure. A plurality of orifices are defined by an orifice plate, with each of the orifices communicating with the fluid reservoir. Ink is forced through the orifices and emerges as a plurality of fluid filaments. Varicosities are generated in the fluid filaments by mechanical stimulation of the orifice plate or by pressure waves which travel through the ink in the fluid reservoir and are coupled to the filaments. The filaments are thereby caused to break up into streams of ink drops of substantially uniform size and spacing.
A plurality of charge electrodes are positioned beneath the orifice plate, with each of the electrodes being adjacent the end of one of the fluid filaments. Charge potentials, selectively applied to the charge electrodes induce corresponding charges on the drops formed from the filament tips. The charged and uncharged drops then pass downwardly through a deflection field, with the charged drops being deflected and the uncharged drops passing unaffected through the field. A drop catcher is positioned adjacent the jet drop streams generated by the print head and cooperates with a deflection electrode to produce a deflection field when a deflection potential is applied between the deflection electrode and the catcher.
A number of problems are encountered at the initiation of operation of an ink jet printer. At start up of such a printer, both the fluid flow through the orifices and the formation of drops from the fluid filaments are extremely irregular and unpredictable. Exceptionally large drops of ink may be formed from the filaments and the trajectories of the drops are relatively uncontrolled. As a consequence, large amounts of ink may be deposited upon the charge electrodes, with the result that the charge electrodes may be shorted.
Several approaches have been taken in order to prevent deposition of ink upon the charge electrode structure at start up of an ink jet printer. In IBM Technical Disclosure Bulletin, Vol. 20, No. 1, June 1977, pp. 33 and 34, a charge electrode structure is shown in which a notched charge electrode plate is pivoted or, alternatively, translated into position adjacent the jet drop streams after start up to reduce wetting of the charged electrodes by the unstable jet drop streams which occur during start up. IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pp. 3216 and 3217, discloses a similar arrangement for laterally shifting a pair of charge electrode plates into and out of their operating positions after start up and prior to shut down, respectively.
U.S. Pat. No. 3,618,858, issued Nov. 9, 1971, to Culp, discloses a notched charge electrode plate which is moved away from a row of fluid filaments at start up until after a stable jet drop stream configuration is obtained. The charge electrode plate is then moved into its operating position in which each of the electrode notches partially surrounds its associated fluid filament. By withdrawing the charge electrode plate from the row of jet drop streams, wetting of the plate and shorting of the charge electrodes is prevented.
U.S. Pat. No. 3,839,721, issued Oct. 1, 1974, to Chen et al, discloses an ink jet printer having a row of nozzles and a plurality of associated U-shaped charge electrodes which are moved into their respective operating positions after start up of the printer and stabilization of the jets.
While movement of the charge electrode structure into operating position after start up of the jets provides a means of keeping the charge electrodes free of ink, the apparatus required for moving the charge electrode structure unduly complicates the printer. Additionally, after repeatedly moving the charge electrode structure into and out of its operating position, the structure may eventually become misaligned with the result that proper charging of the drops in all of the jet drop streams is not accomplished. Finally, in systems where the jet drop streams are stabilized prior to movement of the charge electrodes into their operating positions, it will be appreciated that the drops formed during start up cannot be charged and that these drops will therefore pass downward, striking the print receiving medium.
In order to prevent undue wetting of the print receiving medium at start up, printers such as disclosed in the above identified IBM Technical Disclosure Bulletin, Vol. 19, have incorporated catcher structures which are moved into a position at start up such that they intercept the drops. As shown in U.S. Pat. No. 4,081,804, issued Mar. 28, 1978, to Van Breeman et al, it is known to mount the print head above a drip pan at start up to collect drops formed from the fluid filaments until the jets become stable. A print receiving medium is thereafter positioned above the drip pan, and printing is initiated. While these printers provide a means of handling the drops produced by the unstable fluid filaments at start up, they also require substantial additional structure to dispose of such drops.
Accordingly, it is seen that there is a need for a simple ink jet printer construction in which the deposit of drops on the charge electrode structure from unstable jets at printer start up is prevented and in which fluid from the jet drop streams at start up is prevented from striking the print receiving medium.